In recent years, there have been disclosed techniques for fabricating a light-emitting device such as a light-emitting diode (abbreviated as LED) and a laser diode (abbreviated as LD) from a layer formed of boron phosphide (chemical formula: BP), which is a type of Group III-V compound semiconductor (see, for example, U.S. Pat. No. 6,069,021). A boron phosphide-based semiconductor tends to form a p-type conductive layer, because the effective mass of a hole is smaller than that of an electron (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2-288388). Recently, a light-emitting device is known to have a p-type boron phosphide layer serving as an electrode-forming layer (contact layer) for forming an Ohmic electrode (see, for example, Japanese Patent Application Laid-Open (kokai) No. 10-242567).
Specifically, a conventional p-type electrode formed so as to attain contact with the p-type boron phosphide contact layer which is provided on a light-emitting layer made of a Group III nitride semiconductor is fabricated from a single layer made of a gold (symbol of element: Au)-zinc (symbol of element: Zn) alloy (see the above Japanese Patent Application Laid-Open (kokai) No. 2-288388). Generally, in a conventional boron phosphide-based semiconductor light-emitting device having an electrode also serving as a pad electrode for establishing wire bonding and being provided on a boron phosphide layer, the pad electrode is formed so as to attain contact with a surface of a p-type or n-type boron phosphide layer (see, for example, Japanese Patent Application Laid-Open (kokai) No. 10-242567).
However, employment of a conventional configuration in which a bottom portion of the electrode is caused to be in contact with a surface of the conductive n-type or p-type boron phosphide layer has failed to completely solve the problem that an electric current supplied for driving the light-emitting device (i.e., device operation current) flows in a short circuit manner into an underlying layer from the bottom portion of the electrode. Therefore, in an LED from which emitted light is extracted to the outside via a boron phosphide crystal layer provided on a light-emitting layer so as to form an electrode, there arises a problem of failure to attain diffusion of the device operation current widely in a light emission area. Thus, at present, an increase in emission intensity of a boron phosphide-based semiconductor light-emitting device cannot be fully attained by increasing the light emission area.
In order to overcome the aforementioned drawback involved in the background art, the present invention provides a configuration of a pad electrode for effectively diffusing a device operation current in a wide range of an emission area. Thus, an object of the present invention is to provide a boron phosphide-based semiconductor light-emitting device having such a pad electrode. Another object of the invention is to provide a production method for producing the boron phosphide-based semiconductor light-emitting device.